Ammonia oxidation is carried out industrially with air to generate nitric oxide, which used to make nitric acid (the Ostwald Process) or with air and methane to generate hydrogen cyanide (the Andrussow Process). In both processes, the reactant gases are mixed and passed at elevated temperature and pressure through a reaction vessel in which is placed a pack of platinum/rhodium gauzes that catalyse the oxidation reactions. The gauzes are typically circular and are supported on a frame or basket that holds them perpendicular to the flow of gases through the reactor. The catalyst pack may also comprise one or more palladium-rich gauzes, known as “catchment gauzes” that act to capture volatilised platinum.
In recent years there has been a desire to thrift the amount of platinum used in the catalyst packs and catalyst arrangements combining both precious metal gauzes and particulate metal oxide catalysts have been tested. WO 99/64352 describes a process wherein a mixture of ammonia and air at an elevated temperature is fed to a catalyst comprising one or more gauzes of at least one precious metal in elemental filamentary form, and the resultant gas mixture passed through a layer of a particulate oxidic cobalt-containing catalyst placed below the gauzes.
In addition, the evolution of nitrous oxide (N2O) from ammonia oxidation processes using has come under scrutiny as a potent greenhouse gas, and nitrous oxide abatement catalysts have also been included in the ammonia oxidation vessels as layers underneath the precious metal ammonia oxidation catalyst. For example, it has been proposed in WO 99/07638 to oxidise ammonia by combusting ammonia with air in the presence of a platinum gauze catalyst and passing the resultant gasses through a layer of nitrous oxide decomposition catalyst comprising a ceramic doped with specific metals or metal oxides disposed below the gauzes and above heat recovery means in the ammonia oxidation reactor. Similarly, WO 00/13789 describes a similar process whereby a metal oxide selected from those of La, Cr, Mn, Fe, Co, Ni and Cu was used as a nitrous oxide decomposition catalyst immediately after a platinum gauze in an ammonia oxidation reactor.
In each of these cases the problem exists that by placing a particulate catalyst in a layer below the precious metal gauzes an increase in the resistance to flow of the gases takes place. This increases the pressure drop through the reaction vessel thereby increasing compression costs. Furthermore in processes operated at high pressure, e.g. above 7 barg, the contact time between the reactants and the particulate catalyst may not be sufficient in the thin beds used in an attempt to alleviate the pressure drop problem. Therefore there is a need to provide a catalyst arrangement that overcomes these problems.